This invention relates to masks for producing metal conductive paths between elements of a semiconductive electric circuit and, more particularly, to the construction of a molybdenum mask with an abrasion resistant coating for extending useful life of the mask by inhibiting a development of scratches and other surface imperfections which introduce defects in the conductive paths.
Photolithographic procedures are employed in the construction of semiconductive electric circuits. Typically, a number of circuit modules or chips are supported on carriers which, in turn, are physically supported and electrically connected by a base such as a card or board. In one form of construction, the base is constructed of a ceramic material having electrically conductive paths of metallic material deposited thereon. The base may be fabricated as a laminate comprising a plurality of layers of ceramic material, each of which supports electrically conductive paths which allow for interconnection of terminals of the various chips supported by the base.
The conductive paths are fabricated by photolithographic techniques employing a mask through which material of the conductive paths, in a paste form, is supplied via the mask to the ceramic base in a procedure similar to a silk-screen printing process.
The printing process is done before curing of the clay of the ceramic, while the clay is still soft; thereafter, both the conductive paths of the paste material and the ceramic base are fired together to produce the rigid base with the rigid electrically-conductive paths thereon. The base, prior to the firing, sometimes is referred to as a green sheet, composed primarily of alumina with a small percentage of glass, such as 90% alumina and 10% glass. Such a formulation is compatible with the expansion and contraction characteristics of conductive paths formed of molybdenum.
The ceramic "green sheet" in the unfired condition has a nominal thickness in the range of 0.2 millimeters to 0.28 millimeters, and is a mixture of ceramic and glass powder suspended in an organic binder. The paste for forming the conductive paths may consist of, by way of example, a molybdenum powder uniformly dispersed in a resin and solvent mixture. Metallization of the green sheet is accomplished by extruding the molybdenum paste through a nozzle as the nozzle traverses the mask. The mask is in contact with the green sheet during the extrusion process. The pattern of the conductive paths is defined by the mask.
The mask is formed of metal, such as copper, molybdenum, or stainless steel in the form of a thin foil having a thickness in the range of 0.001-0.006 inch. The mask may have a square or rectangular shape ranging from 0.5 to 7 inches on a side. The masks are fabricated in a composite structure wherein, when viewed from one surface, the mask appears as a mesh or screen while, when viewed from the opposite surface, the mask appears as a stencil having elongated cut-out portions defining the shapes of the conductive paths. The surface with the stencil is placed against the ceramic base during the process of extruding the paste material through the mask upon the base.
A problem arises in that the pressing of the metallic paste through the apertures of the mask introduces a wearing of the mask due to friction between the particles of metal in the paste and the metal of the mask. In this respect, masks fabricated of pure molybdenum have been most successful in retaining dimensional stability of the mask during repeated usage of the mask so as to insure the reproduction of the fine metallic conductive paths on the ceramic base. However, even the molybdenum masks tend to fail after several hundred cycles of repeated use.